As methods for fractionation of an oily substance composed of a mixture of liquid fats and oils, liquid waxes and other similar components by utilizing the difference in melting points of respective components, there have been known, for example, (1) organic solvent fractionation using an organic solvent such as acetone, hexane or the like, (2) detergent fractionation using a surfactant, (3) dry fractionation, i.e., winterization by cooling and (4) sweating.
Among these, the method (1) is advantageous because fractionation can be carried out precisely. However, on the other hand, since a flammable solvent should be used, this is dangerous and requires high production cost. The method (2) has inferior precision of fractionation and its product has inferior quality in comparison with those of the method (1). In addition, separation of a fat or oil from an aqueous solution containing a surfactant and treatment of waste water containing a surfactant are troublesome. The method (3) requires expensive crystallization tank facilities. In addition, productivity, fractionation efficiency and quality of a product are inferior to those of the above two methods. Application of the method (4) is limited to certain kinds of fats and oils. That is, it is employed for removing a wax but is not suitable for fractionation of fats and oils.
The present invention relates to an improvement of the above method (3). Usually, the above method (3) is carried out by lowering the temperature of a refrigeration medium stepwise to deposit crystals slowly in a vertical crystallization tank equipped with a stirrer. In this method, crystals in the form of a slurry are discharged from a bottom part of the tank after crystallization and then pressed or filtered with a belting press or filter press to fractionate into a solid fraction, i.e., crystal fraction and a liquid fraction in a post step. In general, this can be operated under such conditions that a crystallization degree is relatively low such as less that 10 to 30% by weight as the yield of a crystal fraction after pressing based on the total yield of the product. However, when a crystallization degree becomes higher, for example, more than 30%, particularly, 40 to 70% by weight as the yield of a crystal fraction after pressing based on the total yield of the product, a crystalline mass completely loses fluidity and crystals can hardly be discharged from the crystallization tank unless crystallization is carried out with stirring.
Accordingly, in order to obtain crystals in a high yield, it is necessary to carry out crystallization with stirring gently. In this case, the resulting crystals are liable to include a liquid fraction, which results in remarkable deterioration of quality of the crystals. Further, a heavy-duty stirrer is required for stirring, which results in a high production cost.
Due to these reasons, a conventional method is operated in a relatively low crystallization degree such as less than 10 to 30% by weight as the yield of a crystal fraction after pressing based on the total yield of the product. However, a lower melting point fraction can not be sufficiently concentrated because of the low crystallization degree and, thereby, the resulting product has inferior quality.
In the case of fractionation of a non-laurin fat or oil according to the above method (3), it has been proposed to add crystal seeds to a molten fat to accelerate crystallization (Japanese Patent Publication No. 15759/1981 and Japanese Patent Laid Open Publication No. 101197/1985). However, when a crystallization degree becomes 40 to 60% by weight as the yield of a crystal fraction after pressing based on the total yield of the product, such a method is hardly operable because the resulting crystalline mass loses fluidity and is hardly discharged from a crystallization tank.
Under these circumstances, the present inventors have intensively studied means for providing fluidity to a crystalline mass having less fluidity formed in winterization to obtain an improved industrially applicable method for dry fractionation which is operable even in a high crystallization degree. As the results, it has been unexpectedly found that, particularly in non or less-laurin fats and oils such as palm oil, an oily mass formed by crystallization even in a high crystallization degree can be converted into a creamy slurry having a self-fluidity by crushing and kneading the mass with a suitable shearing force, physical vibration or the like, and that the slurry thus treated has remarkably superior filtering characteristics in comparison with an untreated crystalline mass. Further, it has been found that, when a non or less-laurin fat or oil is cooled with a gaseous or liquid refrigeration medium such as air or water to crystallize slowly, fine dendrites of a high melting point fraction grow and a macroscopically globular crystalline agglomerate wherein an amorphous lower melting point fraction is included in the fine crystalline structure of the dendrites is formed, and that such a crystalline agglomerate can be readily separated into a crystal part being rich in the high melting point fraction and an oily part being rich in the low melting point fraction by simply applying a shearing force. Furthermore, it has been found that the globular crystalline agglomerates become a slurry having such a low viscosity that it can be transported by a pump.